CARTILAGE STRESSES IN THE HUMAN HIP-JOINT

The total surface stress measured in vitro on acetabular cartilage whe n step-loaded by an instrumented hemiprosthesis are partitioned into f luid and cartilage network stresses using a finite element model of th e cartilage layer and measurements of the layer consolidation. The fin ite element model is based on in situ measurements of cartilage geomet ry and constitutive properties. Unique instrumentation was employed to collect the geometry and constitutive properties and pressure and con solidation data. When loaded, cartilage consolidates and exudes its in terstitial fluid through and from its solid network into the interarti cular gap. The finite element solutions include the spatial distributi ons of fluid and network stresses, the normal flow velocities into the gap, and the contact net work stresses at the cartilage surface, all versus time. Even after long-duration application of physiological-lev el force, fluid pressure supports 90 percent of the load with the cart ilage network stresses remaining well below the drained modulus of car tilage. The results support the ''weeping'' mechanism of joint lubrica tion proposed by McCutchen.